Abstract

The binuclear copper(II) complex $[Cu_2(RSSR)_2]$ (1), where RSSR is a dianionic Schiff base derived from 2-(thioethyl)salicylaldimine having a disulfide bond is prepared, structurally characterized by X-ray crystallography and its photo-induced DNA cleavage activity studied. The Schiff base ligand $H_2RSSR$ is also structurally characterized. The crystal structure of 1 shows the discrete dimeric nature of the complex with each metal showing square-planar geometry with a CuN2O2 coordination (Cu \dot \dot \dot Cu, 5.011(1) A). The tetradentate Schiff base RSSR acts as a linker of two copper centers. The sulfur atoms in the disul.de unit do not show any apparent interaction with the metal ion. Complex 1, which is cleavage inactive in the dark in the presence of reducing agents, shows signi.cant cleavage of supercoiled pUC19 DNA on exposure to UV light of 312 nm or visible light of different wavelengths under aerobic conditions, in the absence of any additives. DNA cleavage data from control experiments reveal involvement of the disul.de unit as a photosensitizer undergoing photo-induced S-S bond cleavage on exposure to UV light and the resulting species activates molecular oxygen to form singlet oxygen $(^1O_2)$ that causes DNA cleavage following a type-II process. Photo-induced DNA cleavage by 1 on red-light exposure using a Cwlaser of 632.8 nm or a pulsed ruby laser of 694 nm is proposed to involve sul.de radicals in a type-I process and hydroxyl radicals as the reactive species.